The contribution of demographic momentum and component assumptions to projected population change: an extension of the Bongaarts-Bullateo analysis to subnational ethnic groups

Philip H. Rees, University of Leeds
Pia N. Wohland, Newcastle University

The first question that users of population projections ask is what is driving the results presented. Part of the answer lies in the assumption made for future mortality rates, future fertility rates and future migration rates and flows. But future populations also depend on the age-sex structure of the starting populations: it takes 70 to 80 years for the effect of the starting structure to work itself out. The contribution of the existing age structure to the future population is termed demographic momentum. We adapt and extend a methodology used by Bongaarts and Bullateo (1999) to projections in which both international and internal migration components are influential for population sub-groups whose starting age structures are very diverse. The paper reports on the methods, assumptions and results of running a systematic series of scenario projections which enable us work out the drivers of future ethnic population change for the UK’s local areas over the time horizon 2001 to 2051. For the UK population as a whole, between 2001 and 2051, international migration adds 14.8 millions to a population of 59.1 millions. Below replacement fertility subtracts 6.1 millions, while declining mortality adds 5.9 millions and demographic momentum adds 2.2 millions. Internal migration reduces the total population by 0.1 million through shifting populations to local areas with slightly lower growth potential. When we shift the perspective to individual groups, there is a wide range of different outcomes. The largest addition to the White British population is declining mortality followed by immigration; at the other extreme the largest contribution to the Mixed ethnicity groups is from demographic momentum. Full details of component contributions to future population changes in 16 ethnic groups across 352 local areas in England are presented in the paper.